They were used to pin wrap skirts to keep the wind from blowing them open. They made plaid skirts called tartan skirts and they came with one of these pins. Okay, now everyone knows how old I am, lol.
I also have one similar to this. They also gave them out at the public swimming pool to give you the number to your locker where you put your clothes and valuables when you changed clothes at the pool. There was a number attached to this pin and you had to show that to get your things out of our locker when you got ready to leave the pool. You could pin it to your swimming suit strap or suit so you would not lose it while swimming.
When i was a little girl my dad worked at the hospital and he brought one home with all of his keys,he said they used them in the laundry to close up the big bags and they were numbered,more than a dozen uses for them,now & then,fanned& bidding.